1. The Field of the Invention
Exemplary embodiments of the present invention relate to the field of laser devices. More specifically, the exemplary embodiments relate to a segmented detector for performing eye safety measurements on a laser device.
2. The Relevant Technology
Laser devices are used in a variety of applications. For example, laser devices are used as data transmitters in optical networks, providing very high bandwidth and data carrying capabilities. Regardless of the specific application, every application that uses a laser device must conform to some level of eye safety. Ideally, the laser falls within the safest category, which is known as Class 1. Typical fiber optic transceivers are designed to have maximum optical output power levels which meet Class 1 eye safety limits, and are thus safe for unprotected viewing without precautions. This Class 1 eye safety limit must be met under all conditions, including all reasonable single fault conditions, which are defined as reasonable failures of a single component or connection. The specific details of the Class 1 standard are specified in International Electrotechnical Commission (IEC) 60825-1:1993+A1:1997+A2:2001, “Safety of laser products—Part 1: Equipment classification, requirements and user's guide”, Edition 1:1993 with amendments 1:1997 and 2:2001. In order to ensure that this standard is met, the laser output power must be measured using appropriate measuring equipment.
In general, prior art designs ensure eye safety by one of two methods. In the simplest case, the laser and optical system is fundamentally eye safe because the maximum power the device can emit is less than the eye safety limit. This is often the case in longer wavelength lasers that operate in the 1310-1550 nanometer (mu) bands. In other cases, particularly those involving shorter wavelength 850 nm lasers, the eye safety limit is ensured by redundant electrical circuits that monitor either the laser current or, more directly, monitor the laser output power through a monitor photodiode. Redundant systems are required, because the overall monitoring system must continue to function in the event of the failure of a single electrical component or connection.
Unfortunately, the design of short wavelength optical transceivers is often complicated by the fact that the desired normal operating power is often quite close to, if not just below, the eye safety limit. This is true because the maximum data transmission rates for optoelectronic devices occur at the maximum power output. Therefore, designing a system to reliably distinguish between normal and unsafe levels of laser power is challenging. In fact, the standards for acceptable output power are often defined by a minimum value for communications reliability and a maximum which corresponds to the eye safety limit. The desire to have the largest usable output power range will thus tend to make the problem of eye safety more difficult.
One example of a portion of an exemplary laser device package that may require monitoring for eye safety is shown in FIG. 1 and designated generally as reference numeral 10. Package 10 schematically illustrates a cross section of a laser emitter 12 in a housing 14. Housing 14 defines a cylindrical central bore 16 having a first end 13 immediately adjacent laser emitter 12, and a second end 20 located distally from laser emitter 12. Second end 20 can have a flared portion 18 that has an increasing diameter when going from end 13 towards end 20.
Laser emitter 12 has an optical axis 24 that also corresponds to the axis of central bore 16. The central bore 16 is sized and configured to accept a ferrule (not shown) containing, for example, an optical fiber capable of transmitting optical signals from emitter 12 to some remote location. In this example, laser emitter 12 is a vertical cavity surface emitting laser (VCSEL) that operates according to the 10 gigabit per second (Gb/s) standard form factor pluggable (XFP) standard. Bore 16 can then accept a standard plug, such as an LC connector plug. Other types of emitters, data speeds, and plugs are also possible.
Laser emitter 12 transmits a laser beam at a point 25. While, in this embodiment, laser emitter 12 is shown as being contained within housing 14, and transmitting a laser beam at point 25, this need not be the case. Laser emitter 12 can be located at any point behind housing 14, and the laser beam focused to point 25 using, for example, one or more lenses. In either case, point 25 is the apparent source for the laser beam that enters bore 16.
While the laser beam that is transmitted from point 25 is actually one coherent beam, it is perceived by a viewer looking at bore 16, represented by an arrow A, as being divided into two parts. A first part 26 is transmitted directly from point 25 to a point external to bore 16, while a second part 28 is reflected off of an inside wall 16a of bore 16. Unfortunately, this makes it somewhat problematic to measure the output power of laser emitter 12 to verify whether the eye safety limits for Class 1 devices, or any other eye safety limits, are being met.
First part 26 will be generally shaped like a cone. When viewed perpendicularly, this cone will appear as a circle having a specific area. One way to calculate the area of this circle is to use a measurement of the numerical aperture that is defined as the sine of the vertex angle of the largest cone of meridional rays that can enter or leave an optical system or element, multiplied by the refractive index of the medium in which the vertex of the cone is located. The vertex angle is represented in FIG. 1 as angle “B”. In this embodiment, the refractive index of the air is 1. Using standard formulas known to those of skill in the art, the numerical aperture for one example geometric configuration can then be calculated as 0.18.
The problem of measuring the output power of both components of the beam is shown generally in FIG. 2 (not to scale). FIG. 2 illustrates the view looking into the barrel of device 10 from about 100 mm away. Beam 26, which comes directly from point 25, is directed to a point 42 having a subtense of approximately 0.3 milli-radians (mrad). Beam 28, which is reflected off of inside wall 16a, has a subtense of approximately 12.5-12.9 mrad, which is illustrated by the hatched portion of the circular representation of the beams 26 and 28, depending on whether the reflection, and therefore the apparent source, is from end 18 or end 20. To make an accurate determination of eye safety, it is desirable to measure the intensity or optical power of both beam 26 and beam 28 separately.
Currently, making an accurate measurement of both optical powers requires an operator to use expensive optical characterization equipment. Additionally, using such equipment to make the measurements requires a significant amount of time. This corresponds directly to a significant amount of money expended to make these measurements.